The present invention relates to high-speed data communication cables. More particularly, it relates to a high-speed data communication cable with adjustable coupling reactances between the twisted pairs within a cable to establish a known, consistent, and repeatable crosstalk level between the twisted pairs within a cable.
High speed data communications cables in current usage include pairs of wire twisted together forming a balanced transmission line. Such pairs of wire are referred to as twisted pairs.
One common type of conventional cable for high-speed data communications includes multiple twisted pairs within it. In each twisted pair, the wires are twisted together in a helical fashion, thus forming a balanced transmission line. Twisted pairs that are placed in close proximity, such as within a cable, may transfer electrical energy from one pair of the cable to another. Such energy transfer between pairs is undesirable and is referred to as crosstalk. Crosstalk is electromagnetic noise coupled to a twisted pair from an adjacent twisted pair, or from an adjacent cable. Telecommunications systems contain noise that interferes with the transmission of information. Crosstalk increases the interference to the information being transmitted through the twisted pair. The increased interference due to crosstalk can cause an increase in the occurrence of data transmission errors and a concomitant decrease in the data transmission rate. The Telecommunications Industry Association (TIA) and Electronics Industry Association (EIA) have defined standards for crosstalk in a data communications cable that include: TIA/EIA 568-A-2, published Aug. 14, 1998. The International Electrotechnical Commission (IEC) has also defined standards for data communications cable crosstalk, including ISO/IEC 11801 that is the international equivalent to TIA/EIA 568-A. One high performance standard for data communications cable is ISO/IEC 11801, Category 5.
Crosstalk is primarily capacitively coupled or inductively coupled energy passing between adjacent twisted pairs within a cable. Among the factors that determine the amount of crosstalk energy coupled between the wires in adjacent twisted pairs, the center-to-center distance between the wires in the adjacent twisted pairs is very important. The center-to-center distance is defined herein to be the distance between the center of one wire of a twisted pair to the center of another wire in an adjacent twisted pair. The magnitude of both capacitively coupled and inductively coupled crosstalk is inversely proportional to the center-to-center distance between wires. Increasing the distance between twisted pairs can thus reduce the level of crosstalk interference. Another factor relating to the level of crosstalk is the distance over which the wires run parallel to one another. Twisted pairs that have longer parallel runs typically have higher levels of crosstalk occurring between them.
In twisted pairs, the rate of the twist is known as the twist lay, and it is the distance between adjacent twists of the wire. The direction of the twist of a twisted pair is known as the twist direction. Adjacent twisted pairs having the same twist lay and/or opposing twist directions tend to lie more closely together within a cable than if they have different twist lays and/or same twist directions. Thus, compared to twisted pairs having different twist lays and/or same twist directions, adjacent twisted pairs having the same twist lay and opposing directions have a reduced center-to-center distance, and longer parallel run. Therefore, the level of crosstalk energy coupled between the wires in adjacent twisted pairs tends to be higher between twisted pairs that have the same twist lay and/or opposing directions as compared to other twisted pairs that have different twist lays and/or same twist directions. Thus, the unique twist lay serves to decrease the level of crosstalk between the adjacent twisted pairs within the cable. Therefore, twisted pairs within a cable are sometimes given unique twist lays when compared to other adjacent twisted pairs within the cable.
As the continuous twisted or helical structure reaches a termination point, for example as the cable is terminated to be joined to a connector, the helical structures of the individual twisted pairs are deformed to mate with contacts in the terminating hardware creating a de-twisted region within the cable. The actual angle of arrival of the helix of the individual twisted pairs in relation to the mating hardware depends on where the cable is cut within its length. Therefore, the amount of deformation required to align the conductors of the wire pair with the connection points can vary from twisted pair to twisted pair within a cable. The random nature of the deformation of the helical structure creates undesirable inter-pair coupling variations from one connector to the next. Therefore, although the unique twist lay and twist direction can reduce the level of crosstalk within the cable, the de-twisting action produces a level of crosstalk that tends to be random.
In an attempt to reach cross-manufacturer compatibility, EIA/TIA mandates a known coupling level in Category 5 mating hardware. Mating hardware is designed, via counter-coupling, to compensate for the mandated coupling level in order to establish a predetermined level of coupling in a data communications link over a Category 5 cable. The variability in the inter-pair coupling encountered from one plug to the next serves to limit the effectiveness of the counter-coupling compensation.
This specified, standard level of coupling within the mating hardware is provided so that overall the system can have a level of crosstalk that ensures that the particular transmission standard is properly met. Although it is possible to reduce the actual amount of coupling in the mating hardware to improve overall performance, this is not desirable in order to be in compliance with the appropriate standards and reverse compatibility reasons as well. What is preferable is a constant, repeatable and known level of crosstalk. If a Category 5 plug is connected to a superior performance jack, it is expected that the plug and jack will be able to meet Category 5 coupling specifications. This means that the jack/plug must be able to counter-couple for the level of coupling specified for a Category 5 plug/jack. In addition, if two superior performance connectors are used, it is reasonable to expect that the superior performance mating hardware is able to counter-couple for the level of coupling specified for the superior performance hardware.
It is desirable for the crosstalk occurring in the region adjacent to where the twisted pairs have exited from the cable be of a known, consistent, repeatable, and standard value in order to mate with the connecting hardware. At least part of the region is herein referred to as the xe2x80x9cdetwistedxe2x80x9d portion of the cable. Various conventional methods have been used in an attempt to improve the consistency of counter-coupling within the cable and jack or plug. For example, the use of shielded connectors, lead frames, and complex electronic counter-coupling have been used. However, these methods often increase the time required for installation, may require special tools, and can increase the material cost due to a larger parts count. This may lead to market acceptance problems due to the increased costs associated with the special tooling and the additional training required.
The present invention provides an improved method and apparatus for creating consistent, known, and repeatable levels of crosstalk between twisted pairs within a data cable by adjusting the coupling reactances between twisted pairs.
According to one aspect, the apparatus for adjusting the coupling reactances includes a cable having a plurality of twisted pairs. The cable has a de-twisted region where the twisted pairs transition from a twisted configuration to an untwisted configuration and are arranged in a predetermined configuration. An isolation element is located in the de-twisted region of the cable controlling the coupling between adjacent pairs.
In one embodiment, the isolation element may be constructed of a dielectric material, a conductive material, or a ferromagnetic material. In another embodiment, the present invention may also include an isolation element having a window defined therethrough for selectively adjusting the coupling reactances between the twisted pairs within the cable. In another embodiment, the isolation element may have a nonhomogeneous dielectric constant over its length to vary the electrical thickness of the isolation element. Alternatively, the isolation element may vary in its physical thickness over its length, and/or the dielectric constant of the material may vary over its length to vary the electrical thickness of the isolation element. In another embodiment of the present invention, the isolation element may have a pattern of features including gaps for adjusting the coupling reactances between the twisted pairs within the cable.
In another aspect of the present invention a cable having a standard level of crosstalk relative to a conventional cable is disclosed. The cable has a plurality of twisted pairs and de-twisted region where the twisted pairs transition from a twisted configuration to an untwisted configuration and arranged for mating with associated mating hardware. In one embodiment, a means for isolating the two wires comprising one of the plurality of the twisted pairs from the two wires comprising an adjacent twisted pair, and for adjusting the coupling reactances within the de-twisted region of the cable to achieve a desired level of crosstalk between the twisted pairs is disclosed. In one embodiment, the means for isolating may include an isolation element that can have at least one window defined therethrough. The window or windows are sized and arranged for creating and adjusting coupling reactances between the adjacent twisted pairs.
In another aspect of the present invention a terminated cable having a desired level of crosstalk and controlling crosstalk characteristics is disclosed. The cable has a plurality of twisted pairs and a de-twisted region where the twisted wire transitions from a twisted configuration to an untwisted configuration and are linearly arranged. The cable may include a means for creating a larger center-to-center distance between a wire of one twisted pair and a wire of an adjacent twisted pairs. The means for creating a larger center-to-center distance include an isolation element having a varying thickness and/or a varying dielectric constant.
In another aspect of the invention, a cable having a repeatable level of crosstalk terminated with mating hardware includes a plurality of twisted pairs of conductors, that exit from the cable into a first region adjacent to the exit region of the cable, and an isolation element having top and bottom surfaces, and an end region distal to the exit region of the cable, and constructed and arranged to physically separate and at least partially electrically isolate individual twisted pairs from one another, and a second region adjacent to the end region of the isolation element, wherein each twisted pair is detwisted and oriented to electrically mate with the mating hardware.
In one embodiment, the isolation element includes a plurality of main channels on the top surface of isolation element and at least one main channel on the bottom surface of the isolation element, wherein each of the plurality of twisted pairs are disposed within a single main channel. In another embodiment, the main channels have two sub-channels and have a ridge vertically extending between them forming the two sub-channels into a W shape with each sub-channel containing one wire of a twisted pair.
In another embodiment, the isolation element can include a laminated structure with at least first, second, and third layers. In one embodiment, the first layer is a conductor and the second and third layers are dielectric materials. In one embodiment, the first layer is composed of stainless steel, and in another embodiment, the second and third layers are composed of MYLAR(copyright) tape. MYLAR(copyright), as used herein, includes polyester film in general that retains good physical properties over a wide temperature range, has a high tensile tear and impact strength, is inert to water, is moisture-vapor resistant and is unaffected and does not transmit oils, greases, or volatile aromatics. In particular, one form of polyester can be polyethylene terephthalate. In another embodiment, the first layer of the laminated structure is at virtual ground with respect to the plurality of twisted pairs.
In another embodiment, the plurality of twisted pairs of conductors have a distance between adjacent twists of the wire equal to a twist lay and the first region has a length between one-half and one twist lays.